Construction is not rocket science. I wonder what the contractor did that was so wrong.
Could be any single thing, or multiple smaller things:
I don't know any specifics for this location, each project and location is unique due to local soils and anticipated weather conditions.
The General Contractor is responsible for construction and quality of the entire project, each project probably has multiple Sub-contractors.
Depth, diameter and shape of piers might not meet specifications to prevent deep frost line having piers and footing from heaving.
Density and strength of concrete, various add-mixtures required, and they were not.
Concrete could have been too dry or was already setting up when it was poured.
Or it could have been placed during hot weather, and it needed to be cured slower without correct /technique procedures.
Specifications might have required gravel or sand at bottom of hole prior to placing concrete, and none was used.
Might have required/specified pier casings and none were used -- especially if the concrete top is to be above the adjacent ground level.
Might have used incorrect reinforcing bars, or insufficient ties.
Might have required dowels or dimples to be used on steel sleeves. for proper adhesion.